The present invention relates to non-volatile memory (NVM). More particularly, this invention relates to non-volatile memory cells fabricated using an application specific integrated circuit (ASIC) or conventional logic process. In the present application, a conventional logic process is defined as a semiconductor process that implements single-well or twin-well technology and uses a single layer of polysilicon. This invention further relates to a method of operating a non-volatile memory to ensure maximum data retention time.
For system-on-chip (SOC) applications, it is desirable to integrate many functional blocks into a single integrated circuit. The most commonly used blocks include a microprocessor or micro-controller, static random access memory (SRAM) blocks, non-volatile memory blocks, and various special function logic blocks. However, traditional non-volatile memory processes, which typically use stacked gate or split-gate memory cells, are not compatible with a conventional logic process. The combination of a non-volatile memory process and a conventional logic process results in much more complicated and expensive xe2x80x9cmerged non-volatile memory and logicxe2x80x9d process to implement system-on-chip integrated circuits. This is undesirable because the typical usage of the non-volatile memory block in an SOC application is small in relation to the overall chip size.
There are several prior art approaches to minimize the complexity of such a merged non-volatile memory and logic process. For example, U.S. Pat. No. 5,879,990 to Dormans et al. describes a process that requires at least two layers of polysilicon and two sets of transistors to implement both the normal logic transistors and the non-volatile memory transistors. This process is therefore more complex than a conventional logic process, which requires only a single layer of polysilicon.
U.S. Pat. No. 5,301,150 to Sullivan et al. describes a single poly process to implement a non-volatile memory cell. In this patent, the control gate to floating gate coupling is implemented using an n-well inversion capacitor. The control gate is therefore implemented using the n-well. An injector region must be coupled to the inversion layer in the n-well. The use of an n-well as the control gate and the need for an injector region result in a relatively large cell size.
U.S. Pat. No. 5,504,706 to D""Arrigo et al. describes a single poly process to implement a non-volatile memory cell that does not use an n-well as a control gate. FIG. 1A is a schematic diagram illustrating an array of non-volatile memory cells C00-C11 as described by D""Arrigo et al. FIG. 1B is a cross sectional view of one of these non-volatile memory cells. As shown in FIG. 1A, each of the memory cells contains a transistor 24 having a source connected to a virtual-ground (VG) line and a drain connected to a bit line (BL). The transistor 24 further has a floating gate 40 which is coupled to a word line (WL) 86 through a coupling capacitor. The coupling capacitor includes n+ region 80 (FIG. 1B), which is located under the floating gate 40 and which is continuous with the diffusion word line 86. The capacitance of the coupling capacitor is significantly larger than the gate capacitance of the transistor to allow effective gate control of the transistor from the WL voltage levels. The n+ region 80 is formed by an additional implant to ensure good coupling during operations. This additional implant is not available in a standard logic process. The memory cells 24 are located inside a triple-well structure. More specifically, the memory cells are formed in a p-well or tank 78, which in turn, is formed in an n-tank 76, which in turn, is formed in p-well or substrate 74. A p+contact region 88 is located in p-tank 78, and an n+ contact region 90 is located in n-tank 76. The triple-well structure allows flexibility of biasing in operating the memory cell. More specifically, the triple-well structure allows a large negative voltage (typically xe2x88x929 Volts) to be applied to the word line 86 (i.e., the control gate). Both the extra n+ implant and the triple-well are not available in a conventional logic process. Similarly, U.S. Pat. No. 5,736,764 to Chang describes a p-channel metal-oxide-semiconductor (PMOS) cell having both a select gate and a control gate, wherein additional implants are required underneath the control gate.
In addition, the above-described non-volatile memory cells use a relatively thick tunneling oxide (typically 9 nanometers or more). Such a thick tunneling oxide is not compatible with conventional logic processes, because conventional logic processes provide for logic transistors having a gate oxide thickness of about 5 nm for a 0.25 micron process and 3.5 nm for a 0.18 micron process.
Conventional non-volatile memory cells typically require special high voltage transistors to generate the necessary high voltages (typically 8 Volts to 15 Volts) required to perform program and erase operations of the non-volatile memory cells. These high voltage transistors are not available in a conventional logic process. These high voltage transistors are described, for example, in U.S. Pat. No. 5,723,355 to Chang et al.
U.S. Pat. No. 5,761,126 to Chi et al. describes a single poly electrically programmable read only memory (EPROM) cell that utilizes band-to-band tunneling in silicon to generate channel hot-electrons to be injected into a floating gate from a control gate. A relatively thin tunnel oxide can be used in this memory cell because of the enhanced electron injection. However, this memory cell only supports programming (i.e., electron injection into the floating gate). No support is provided to remove electrons from the floating gate (i.e., an erase operation is not supported).
The use of a thin gate oxide as tunneling oxide presents a challenge for achieving acceptable data retention time for non-volatile memory cells. A thin gate oxide is defined herein as a gate oxide layer having a thickness in the range of 1.5 nanometers (nm) to 6.0 nm. Although programming voltages may be reduced by the use of a thin gate oxide, the thin gate oxide will exacerbate cell disturbances. That is, the thin gate oxide will significantly increase the probability of spurious charge injection or removal from the floating gate during normal program, erase and read operations. This is due to the high electric field present in or near the thin gate oxide. As conventional logic processes scale down in geometry, the gate oxide thickness scales down proportionally. For example, a 0.25 micron process uses a 5 nm gate oxide thickness, a 0.18 micron process uses a 3.5 nm gate oxide thickness, and a 0.15 micron process uses a 3 nm gate oxide thickness. As a result, data-retention becomes a serious problem when using the standard gate oxide as the tunnel oxide in a non-volatile memory cell. U.S. Pat. No. 5,511,020 to Hu et al. describes data refreshing techniques to improve data retention time using very thin tunnel oxides.
It would therefore be desirable to implement a single-poly non-volatile memory cell using a conventional logic process, without requiring process modification and/or additional process steps.
It would also be desirable to have a method of operating non-volatile memory cells in conjunction with volatile memory arrays in a manner that minimizes disturbances from write, erasing and read operations, thereby improving the data retention time for the non-volatile memory cells.
Accordingly, the present invention provides a non-volatile memory cell fabricated using a conventional logic process. The non-volatile memory cell uses a thin gate oxide (i.e., 1.5 nm to 6 nm) available in a conventional logic process. The non-volatile memory cell can be programmed and erased using relatively low voltages. The voltages required to program and erase can be provided by transistors readily available in a conventional logic process (i.e., transistors having a breakdown voltage in the range of 3 Volts to 7 Volts).
In one embodiment, the non-volatile memory cell includes a p-type semiconductor substrate and an n-well located in the substrate. A PMOS transistor is fabricated in the n-well. The PMOS transistor includes the thin gate oxide and an overlying polycrystalline silicon gate. An NMOS capacitor structure is fabricated in the p-type substrate. The NMOS capacitor structure includes an n-type coupling region located in the p-type substrate. The n-type coupling region is formed by the n-type source/drain implants, thereby eliminating the need for any additional implants not normally provided by the conventional logic process. The thin gate oxide and the polycrystalline silicon gate extend over the p-type substrate and the n-type coupling region, thereby forming the NMOS capacitor structure. The NMOS capacitor structure and the PMOS transistor are sized such that the NMOS capacitor structure has a capacitance larger than a capacitance of the PMOS transistor. Advantageously, a triple-well structure is not required by the present invention.
The present invention incorporates a negative voltage generator that provides a negative boosted voltage having a voltage level that is less than the Vss supply voltage by a voltage that is less than a diode turn-on voltage (0.7 Volts). In one embodiment, the negative boosted voltage has a value of xe2x88x920.5 Volts. The negative boosted voltage is applied to the control gate of the non-volatile memory cell to enhance the electron removal operation and normal read operation without requiring a triple-well underneath the control gate.
The present invention also incorporates a positive voltage generator that provides a positive boosted voltage having a voltage level that is greater than the Vdd supply voltage by a voltage that is less than a diode turn-on voltage (0.7 Volts). In one embodiment, the positive boosted voltage has a value equal to Vdd +0.5 Volts. The positive boosted voltage is applied to the N-well of the non-volatile memory cell and the control gates of non-selected memory cells during normal read operations to suppress leakage currents through those non-selected memory cells and to improve operating margins.
In accordance with one embodiment of the present invention, non-volatile memory cells are used in a system-on-a-chip system. After power-up of a system-on-a-chip integrated circuit incorporating the embedded non-volatile memory cells, the contents of the non-volatile memory cells are read out and stored (with or without data decompression operations) into on-chip or off-chip volatile memory. The data contents of the non-volatile memory cells are then refreshed (through charge injection and removal) with optimum signal condition. The non-volatile memory cells then remain in an idle or standby mode substantially with out a significant external electric field. If a reprogramming operation or a refresh operation is required, then the non-volatile memory cells are reprogrammed or refreshed as required and then returned to the idle or standby mode. As a result, the storage characteristics of the thin oxide non-volatile memory cells are improved.
The present invention will be more fully understood in view of the following description and drawings.